Cycles: Is there a way to find the angle between incidence and reflection rays?
The angle between incidence and reflection ray depends on the surface:


If the surface is glossy with roughness 0 (perfect mirror), this angle is double the angle between incidence and surface normal:


If the surface is anything else, the reflected rays scatter based on probability - based on BSDF function (rough surface, diffuse surface,..). You would have to look inside Cycles code to trace these rays individually. The best help you will get directly from devs on IRC channels or through mail-list, look here who owns the code: Module owners.